A conventional battery pack described in PTL 1 includes a battery group in which a plurality of secondary batteries are connected in series, current/voltage detector 4 that detects a voltage corresponding to a charging and discharging current flowing in the battery group and a voltage at each secondary battery in a time-division manner by switching the voltages with a multiplexer, and a controller/operator that acquires and operates the voltages detected by current/voltage detector 4. At this point, the voltage at each secondary battery and the voltage corresponding to the current are sequentially measured.